* Boeing seen launching 777X with around 250 orders, 4 buyers
* Airbus fights back to avoid rare air show defeat
* Boeing says union vote on 777X assembly won't deter launch
* Emirates airline seen in talks to buy 20-25 A380s -source
* UK's Cameron pays lightening visit on eve of show
DUBAI, Nov 17 (Reuters) - Dubai flag carrier Emirates will buy more Airbus A380 superjumbos, the airline's chairman said at the opening of the Dubai Airshow on Sunday, adding total plane orders would exceed the $18 billion agreed at the last event in 2011.
The airline is the largest customer of the A380s, with a current order of 90 jets. "We will order A380s," Sheikh Ahmed bin Saeed Al Maktoum, the chairman of Emirates, told Reuters. "Yes, for sure. We (Emirates) will exceed" the previous total round of orders.
Gulf airlines are set to place orders worth over US$100 billion (S$124. 7 billion) at the Airshow, industry sources said, led by record investment in a new Boeing jet.
The birth of a revamped successor to Boeing's 777 jet, with some 250 orders booked or in the pipeline from four airlines, marks a new step in the growth of Gulf airlines and a fierce contest between Boeing and Airbus for their business.
Boeing is expected to launch the 777X jet with orders from the top three Gulf carriers - Dubai's Emirates, Abu Dhabi's Etihad Airways and Qatar Airways - and Germany's Lufthansa which has already tentatively committed to buy 34 of the planes.
Gulf airlines have been expanding rapidly as they seek to take advantage of a strong geographical position connecting East and West.
A fifth, Hong Kong's Cathay Pacific, could join the programme later, industry experts said, while British Airways has expressed interest in the jet.